心电图（心电图）信号的采集与分析

The sinoatrial node in the right atrium acts as the heart's pacemaker, initiating electrical signals that cause atrial contraction, recorded as the P wave. These signals then pass across the ventricles, causing them to contract and pump blood, recorded as the QRS complex. Finally, ventricular relaxation is recorded as the T wave. This cycle repeats with each heartbeat.

A biopotential amplifier contains three stages: the patient protection stage using resistors and diodes to limit current and ensure safe direction of flow; the instrumentation amplifier with three operational amplifiers to amplify the difference between electrode inputs; and the high pass filter to reduce noise from patient movement or respiration by filtering out low frequency signals.

The QRS complex is much larger than the P wave because the ventricles have significantly greater muscle mass than the atria. The larger ventricles generate stronger electrical signals during contraction. This size difference also causes the ventricles' electrical activity to mask the relaxation of the atria on the ECG recording.

Use a peak detector to identify the R wave peaks in the filtered ECG signal. Determine the positions of two consecutive R peaks using an index array function. Subtract the lower peak position from the higher position and multiply by the sampling period to find the time between heartbeats. Convert this interval to beats per minute by dividing 60 seconds by the interval duration.

A low pass filter with a cutoff frequency of 100 hertz removes high frequency noise using Butterworth or Chebyshev functions. A bandstop filter with cutoff frequencies around 55 and 70 hertz eliminates 60 hertz interference from electrical equipment. Together, these filters attenuate at least 60 decibels per decade in the stop band, revealing distinct P, QRS, and T wave complexes.

A Fast Fourier Transform algorithm calculates and plots the frequency spectrum of an ECG signal, displaying frequency as discrete values on the horizontal axis. This analysis reveals that most signal energy is at low frequencies, while high intensity peaks in the medium frequency range typically represent noise. This information guides filter design to remove unwanted frequencies while preserving cardiac signal components.

ECGs are non-invasive, relatively inexpensive, and accessible tools for assessing heart function and blood flow to the organ. They can diagnose disease, detect abnormalities, and monitor patients with conditions like Acute Coronary Syndrome using 12 leads to identify transient myocardial ischemia in asymptomatic patients. Signal processing methods used in ECGs also extend to other biomedical applications like combined SPECT and CT imaging to visualize cardiac functionality.